A pressure volume regulator for use subsea for supply and pressure control of the barrier fluid supply for pumps and compressors, is described and illustrated in Norwegian patent publication NO 322 566. The objective of a pressure volume regulator is to control supply and dump of lubrication fluid, also termed barrier fluid, as required to secure an overpressure in the motor cavity. More specifically, the barrier fluid in the motor cavity is to be held at some overpressure in relation to the pump or compressor pressure, in order to eliminate flow in the direction from the pump or compressor to the motor. The regulator is typically arranged with one supply line of barrier fluid connected to an inlet, a first outlet connected to a motor compartment and a second outlet connected to a pump or compressor compartment. The pressure of the motor compartment is to be maintained at a higher level than the pressure of the pump or compressor compartment. The leakage path of barrier fluid in a motor/pump unit will typically be through the shaft seal separating the motor and pump/compressor cavity. By maintaining some overpressure of the fluid in the motor cavity, leakages can only flow towards the pump/compressor. Accordingly, the fluid in the motor cavity will act as a barrier against contamination by particles and contaminating fluid which else could leak from the pump or compressor into the motor and shorten the service life of the motor. The barrier fluid also acts as a coolant for the motor and lubricant for the bearings.
The pressure volume regulator of NO 322 566 comprises one inlet and at least two outlets. The pressure is reduced for each subsequent outlet, in the direction of flow from the inlet, by having a pressure controller, such as a pre-tensioned check valve or pressure control valve between each outlet, opening at a predetermined value controlled by the pre-tensioning. If the pressure difference between an upstream outlet and a downstream outlet exceeds a predetermined value, the valve opens. The valve element area on which the inlet flow pressure acts is of the same order of magnitude as the valve element area on which the outlet flow pressure acts.
There is a possibility for having both the inlet flow supply valve and the check valve on the second or last outlet open at the same time. This may result in a continuous dumping of the barrier fluid. Even though this kind of functional failure require a very stiff or incompressible volume downstream the second or last outlet, it is crucial to eliminate the risk for this to happen.
Also, it would be beneficial if the reliability of the pressure volume controller could be improved even further.
The objective of the present invention is to provide a pressure volume regulator that eliminates the failure mechanism mentioned above and also provides improved reliability and a prolonged service life.